The present invention relates to bent pipes suitable for use in piping arrangement for transporting materials containing solids, and to a process for producing the bent pipes.
Solid material transport systems having a pipe for passing therethrough a solid material such as oil sand, coal, ore, sand, earth or municipal refuse have the pipe inner surface thereof exposed to a severe abrasive environment and therefore need to have a sufficient wear resistance over the pipe inner surface. This need increases all the more especially in bent pipes. High Cr cast iron which is excellent in wear resistance has theretofore been used favorably as a material for such pipes.
However, since the high Cr cast iron is low in weldability, pipes of this material cannot be joined to one another by butt welding as required for providing piping systems.
Accordingly, as shown in FIG. 3, a pipe 10 of double-layer structure has been proposed and placed into use which comprises an inner layer 11 of high Cr cast iron and an outer layer 12 of carbon steel or the like which has high weldability. This double-layer pipe 10 is produced by centrifugal casting. After casting the outer layer 12, the inner layer 11 is cast, whereby the inner layer is metallurgically joined with the outer layer to provide a metallurgically integral structure of the two layers.
When such double-layer pipes 10 are used to provide a piping system, one pipe is joined directly to another by butt welding W1 at their outer layers 12, or a flange 13 is welded as at W2 to the outer layer 12 of each of pipes, and the flanges are attached to each other to form a joint for connecting the pipes.
In constructing piping systems, there arises a need to use bent pipes having bent portions of various shapes, such as elbows, U-shaped pipes and S-shaped pipes.
Bent portions can be formed in pipes typically by high-frequency bending work. However, no report has been made on bent pipes of high Cr cast iron formed by high-frequency bending work since high Cr cast iron is brittle and therefore susceptible to cracking when subjected to the bending work. For this reason, it has been thought that the double-layer pipe having an inner layer of high Cr cast iron cannot be worked by high-frequency bending.
To produce bent pipes of a high Cr cast iron to be used in pipe arrangement for transporting solid materials, therefore, a mold of a specified shape is prepared, a bent pipe member is produced by stationary casting, and a flange of carbon steel or like material having high weldability is attached to an end of the pipe member by insert casting. However, this method is not only inefficient and costly, but also has a problem with respect to the reliability of quality of the product in that the pipe produced by stationary casting is likely to have casting defects such as shrinkage cavities unlike centrifugally cast pipes.
We have accomplished the present invention based on the finding that the double- layer pipe described and produced by centrifugal casting can be subjected to high-frequency bending work under optimum conditions.
The present invention relates to a bent pipe for passing therethrough a material containing solids, the bent pipe being formed by subjecting to high-frequency bending work a straight blank pipe prepared by centrifugal casting and having a plurality of layers, the straight blank pipe comprising an outer layer made of a steel having high weldability, and an inner layer made of a high Cr cast iron containing at least Cr in an amount of 10 to 35 wt. % and having high wear resistance, the outer layer and the inner layer being metallurgically joined.
The present invention relates also to a process for producing a bent pipe for passing therethrough a material containing solids, the process comprising a step of preparing by centrifugal casting a straight blank pipe comprising an outer layer of a steel having high weldability and an inner layer of a high Cr cast iron having high wear resistance, the outer layer and the inner layer being metallurgically joined, and a step of forming the bent pipe by subjecting the straight blank pipe to high-frequency bending work, the high-frequency bending work being performed by raising the temperature of the straight pipe at a rate of 50-250xc2x0 C./min and heating the straight blank pipe at a temperature of 1000 to 1050xc2x0 C. by high-frequency heating, bending the straight blank pipe at a rate of 0.3-0.8 mm/sec in the same temperature range, and thereafter cooling the resultant bent pipe at a rate of up to a maximum of 50xc2x0 C./min.
Preferably, the straight blank pipe prepared by centrifugal casting has a barrier layer formed between the outer layer and the inner layer for preventing an alloy component of each of the layers from diffusing into the other layer. It is desired that the barrier layer be about 10 to about 100 xcexcm in thickness.